It is known to oligomerize lower olefins, particularly C2 to C6 olefins, to produce higher molecular weight olefinic products useful as, for example, fuels and precursors in the production of plasticizers, surfactants, and freeze point depressants for lubricating oils. Typically, known oligomerization processes involve contacting a lower olefin with a solid acid catalyst, such as phosphoric acid or, more recently, a crystalline molecular sieve.
For example, U.S. Pat. Nos. 3,960,978 and 4,021,502 disclose the conversion of gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins, into an olefinic gasoline blending stock by contacting the olefins with a ZSM-5 type zeolite. In addition, EP-B-746,538 discloses oligomerization of propene and butene to produce enhanced yields of the trimer using zeolites of the structure types MFI, TON, and MFS, such as ZSM-5, ZSM-22 and ZSM-57.
It is, however, found that commercially available olefinic feedstocks cause rapid deactivation of existing oligomerization catalysts and give a lower selectivity to the trimer than might be expected from the disclosure of EP-B-746,538. Although the reason for these observations is not fully understood, it is believed that the presence of certain sulfur compounds is at least partly responsible for the decrease in activity and selectivity. In particular, it is believed that low molecular weight, aliphatic thiols, sulphides and disulphides are especially troublesome, for example dimethyl, diethyl, and ethyl methyl sulphides, n-propane thiol, 1-butane thiol and 1,1-methylethyl thiol, ethylmethyl and dimethyl disulphides, and tetrahydrothiophene. There is therefore a need for an oligomerization process which exhibits improved catalyst life when used with commercially available olefinic feedstocks.
U.S. Pat. No. 5,157,201 discloses a process for oligomerizing a sulfur-containing C2-C4 olefin feed over a phosphoric acid catalyst, in which the feed is initially passed through a bed of adsorbent at 50 to 175° C. and in the absence of added hydrogen. The adsorbent is at least one metal oxide selected from cobalt oxide, nickel oxide, molybdenum oxide, zinc oxide and copper oxide. The metal oxide is said to be effective in adsorbing 80 to 99% of the sulfur in the feed.
WO 01/30941 discloses an olefin oligomerization process in which a sulfur-containing olefinic hydrocarbon feedstock is contacted with a hydrotreating catalyst in the absence of hydrogen and in the liquid phase. The catalyst typically comprises mixed nickel and molybdenum oxides or mixed cobalt and molybdenum oxides on an alumina support. The catalyst oligomerizes the sulfur compounds as well as the olefins in the feedstock so that sulfur-containing feedstocks can be treated without the need for an initial adsorption step and without rapid reduction in catalyst activity.
According to the present invention, it has now been found that, by prior treatment with a metal oxide at a temperature in excess of 150° C., sulfur-contaminated olefinic feedstocks can be selectively oligomerized over molecular sieve catalysts with a marked increase in the lifetime of the oligomerization catalyst as compared to the same process without the pretreatment step.